1. Field of the Invention
The present invention relates to the production of a layer of polyurethane of high optical quality, which can be used in the production of laminated safety windows, in particular for the production of a layer of thermohardening polyurethane having the properties of resistance to scratching and abrasion, which can be used as a cover layer of a solid or laminated rigid support made of glass and/or a rigid plastic material such as polycarbonate, methyl polymethacrylate, or as a cover layer for a more flexible layer, for example a layer of polyurethane with energy absorption properties, a layer of polyester, these layers themselves being in contact with the rigid support.
2. Description of the Background
A cover layer of the type described above is discussed, for example, in the publications of French Patents 2,187,719; 2,251,608 and 2,574,396 (U.S. Pat. No. 4,657,796). This said layer, which is also self-cicatrizing, is based on thermohardening polyurethane and has a high capacity for elastic deformation, a low modulus of elasticity, less than 2000 DaN/cm.sup.2, and an elongation to rupture of more than 60% with less than 2% plastic deformation, under normal temperature conditions. A thermohardening polyurethane which is especially preferred is the one described in the publication of the French Patent 2,251,608.
In order to produce a layer of this type, a process with reactive flow is generally used, which is a process in which the layer of thermohardening polyurethane is formed starting with a homogeneous mixture of the reaction components, which are allowed to flow continuously on a support, generally a flat support made of glass, which is located below the flow apparatus. A process of reactive flow is described, for example, in French Patent 2,442,128. The components which form the reaction mixture for flow are, on the one hand, a polyol component which is generally a polyether polyol or a polyester polyol with a functionality greater than 2, generally 3 or between 2 and 3, and, on the other hand, an isocyanate component which can be chosen from among 1,6-hexane diisocyanate biurets or triisocyanurates, this component having a functionality of 3.
This process with reactive flow of the mixture of the two components described above results in a layer with excellent optical quality, which also presents the desired properties such as resistance to scratching, to abrasion, to outside agents, to solvents and the like. However, this reactive flow is not entirely satisfactory except at layer thicknesses generally above 200 .mu.m. For lesser thicknesses, on the order of 100 .mu.m and less, although the mechanical properties are generally suitable, the optical quality of the layer obtained is not always satisfactory.
To produce layers of thermohardening polyurethane with low thicknesses, particularly below 300 .mu.m, a process of reactive pulverization has been proposed. In this case, the reaction mixture is no longer allowed to flow using a flow head, but rather pulverized, for example using a device with a bowl rotating at high speed, such as described, for example, in European Patent 0,161,184 (U.S. Pat. No. 4,749,586). But here again, the optical quality obtained is not always satisfactory for layers with a thickness on the order of 100 .mu.m and less.